Portable radiation survey instrument



y 29, 1952 H. v. HERNDON ETAL 2,605,429

PORTABLE RADIATION SURVEY INSTRUMENT Filed Sept. 11, 1950 IN VEN TOR.j/o wa I-cz \Khkrnabn BY Raine 6- Hoff Patented July 29, 1952 rortrABLERADIATION SURVEY INSTRUMENT Howard'V. Herndon and Rodney G. Hoff, Rich-:land, Wash .assignors to the United States of America as represented bythe Unitedfstates Atomic Energy Commission i Application septmb'er 11,1950,serialNol sgdrf 7 Claims. '((11. 250-83.6)

The present invention relates to radiation measuring devices utilizingionization chambers, and more specifically to improvements upon suchradiation measuring devices to reduce the responsetime of the device.

One approach to the problem of reducing the response time of a"radiation measuring device utilizing an ionization chamber is disclosedin Patent'No 2 ;465,938', issued to Francis R.Shonka,

entitled Radiation Measuring Device. The device disclosed in thispatent, however, is only broadly relevant to the present invention andhas certain disadvantages overcome by the present invention. 'For abetter understanding of the invention, reference is made to the singlefigure of the drawing, which is 'a schematic electrical circuit diagramof a. radiation measuring device.

The device consists of an ionization chamber circuit',-and a directcurrent amplifier and measuring circuit which areinterconnected in anovel manner. The ionization chamber circuit consists of an ionizationchamber 9 connected in series with a portionof battery 11, battery l9,resistor l4 and ionization resistor [3. The direct current amplifier andmeasuring circuit consists of a two-stage direct current amplifierincluding a microammeter 12-9 in the output circuit .of the second stagewhich is-ealibrated directly in termsof 'radiationtintensity.

The first stage of the electronic measuring circuit consists of a'vacuum tube t2 having a grid-to-cathode input circuit and aplate-tocathode output circuit. The control grid 23 of the vacuum tube I2 is connected to the center electrode ID .of the ionization chamber 9.The input circuit consists of the ionization resistor I3, a portion ofbattery J 1, resistor l4, battery It and variable .resistor 15, allconnected in series between the control grid23 and the cathode 22 ofvacuum tube 12. Battery 1 6 and battery H are connected in opposition,and hence their voltages tend to cancel so that the control grid 23 hasa proper bias voltage applied thereto. The output circuit of the firststage also includes resistor l4, battery l6 .(which constitutes theplate voltage supply) and. variable'resistor f5, connected in that orderbetween the plate 2! and the cathode 22 of the vacuum tube I2.

The second stage of the amplifier and measuring circuit includes avacuum tube I 8, a cathode-.to-grid input circuit, and a plate-tocathodeoutput circuit. The cathode 2? of vacuum tube [8 is directly "connectedto theca'tho'de 22 of vacuum tube l2. Hence, the input circuit i2"consists of variable resistor [5, battery [6, re-

sistor I4, and a portion-ofbattery l'1, connected in that-order betweenthe cathode 2'! and the I control grid 26 oi vacuum tube 18-. Theportion of battery I! that is connected in the inputcircui-t isadjustable, so that a proper bias voltage may be placed on grid 26. Theoutput circuits consist of resistor 28, battery [9, battery It, andvariable resistor 5, series connected in that order between the plate"25" and=the cathode '21 of vacuum tube l8; 'Hen'ceyit is clearthatvariable resistor T and'ba'ttery l6 are'connected in series with theoutput circuits of both stages of the electronic measuring circuit. Amicroammeter 29 is connected'between the plate 250i vacuum tube 18 andthe negative terminal of battery l9 through the -rheostat 33, which =isused to adjust the current through the cir'cuitt'o a reference valve Theouter electrode-l tor the ionization chamber 9 isconnectedto-thefpositive terminal of battery 19, thus completing theionization chamber circuit through resistor 14, a portion of battery 11,and ion'izati'onresistor l3. The filaments 30 and 31 of vacuum tubes l2and I8 are connected in parallel "a'crossthe' filament supply battery20'.

The instrument may 'be constructed using a type VX1323 for vacuumtube'liz; and a tri'ode connected type 'CK502AX for tube [8. Batteries 6and I9 may be 45 volts, andbattery I 1 may be 15 volts- Ionizationresistor 13 may be of the magnitude :of 10 ohms, "and'the effectivevalue of'resistors 14, 1.5, and .28 may be approximately 1.81megohms,4,000 ohms "and 300,000 ohms, respectively. A meter measuringrfullscale'with a current of 20 n1icroamperes is satisfactory for meter 529;

The filaments 30 Land 3| are heated by the currentsupp'liedby battery20. The plate 2| or" vacuum tube l2 is'place'd at apo'sitive potentialwith respect tothe cathode 22 as a result of the electromotive forcesupplied by battery It, and hence current'm'ay be "madeto flow betweenthe plate 2| and the cathode-"22 in vacuum tube l2. The plate 25 ofvacuum tube I8 is placed at a positive potential as a result of theelectromotive forces of batteries l6 and [9, which are series connected,and hence current may be made to flow from the plate 25 to the cathode21 in vacuum tube I8. The'outer electrode I l of the ionization chamber9 is placed at a positive potential with respect to the'inner electrode10, as a result of the electromotive force supplied primarily bybatteries l1 andlfl, which are connected in a series aidingrelationship. Proper output circuit of vacuum tube l2 causes an;in-. 1crease in the potential drop across resistors i and [5. across resistorsl4 and I5 appears as negative feedback in the grid-to-cathode inputcircuit of vacuum tube 12, so that this increase in potential Thisincrease in the potential dropis thus clear, that the voltage dropimpressed across resistor l4 may be made to exceed 63 per cent of thevoltage developed across resistor [3 as a result of the ion currentflowing therethrough, because of the positive feedback applied to theinput circuit across variable resistor l5. v

By adjusting the positive feedback developed across variable resistor[5, the voltage impressed across resistor I4 may be made equal to thevoltage induced across ionization resistor l3. This may be accomplishedby adjusting the resistance value of variable resistor l5. Under theseconditions, the voltage applied to the ionization chamacross resistorsl4 and I5 as a result of the ionization can never exceed the percentage-trw of the voltage developed {by the ion jcurrent through resistor 13,where a is the amplification factor of the vacuum tube J2. For a VX3213type tube,theratioofi Q is approximately 63 per cent.

Since the cathode 21 of; vacuum tube 18, flS directly connected tothe-cathode, 22 ofvacuum;

tube l2; the cathode 2lwil1 receive-a;positive charge as a result of theincreased voltage drop across resistors l4 and I5; :Thisineifectdrivesthe grid '26 of vacuum tube I8 negatively-with respect to the cathodeZl,and causes a decrease in the plate current flowing through vacuum tubeIS. The resulting; decrease in voltage drop across resistor 28 changes,the magnitude of theicurrent flowing through meter, 29 which-is ameasure ment of the radiation, entering the ionization chamberSL H Thedecrease in current through the output circuit'of vacuum; tube It also,causes a decrease in the potential; drop across variable resistor Sincethe decrease in current in .the'out'put' circuit of vacuum tube [8 ismuch reater than the increase in current in the output circuitofthevac-,

uum tube I2,,the;net result is that. the potential drop across variableresistorl5 is'decreas'ed.

the amplifier throughtthe mechanism of reducing the potential drop"across, variable resistor. Il5..

This feedback isppositive in nature, and thus decreases the negative;feedback that" is developed across resistor 14. However, the potentialdrop across resistor 14 must always exceed therpotential drop acrossresistor l5 in order to apply a negative charge to the grid 26jofvacuum'tube I8.

It is apparent that the first amplifier stage;

constitutes a cathode follower, and that the total change in voltagedeveloped across resistors [.4 and I5 can neverexceed the-ratiov of V Ineffect, a portion of the output voltage 'of=the two-stage amplifier isfed back to the input of ber 9 has remained substantially constant,since the voltage impressed across ionization resistor l3 has beenbucked out by the increase in the potential drop across resistor [4.Thus the ionization chamber 9 does not change in potential,

and the instrument is able to give a reading of the ionization occurringin the ionization chamber without the necessity of waiting forthepartialdischarge of the chamber capacity through the ionization resistor it tobe accomplished. Hence, the time required by the instrument to respondis exceedingly short; and is notl overnedby the product of thecapacitance-of the ionization chamber and its associated circuit, andthe resistance of the ionization resistor 13.

The use of negative feedback to reduce the response time of a radiationmeasuring instrument forms no part of the present-invention, that'havingbeen disclosed in application ,Serial No. 172,546 of Barton L. Weller,filed Julyfl; 1950. However, heretofore the use of negative'feedbackcircuits to reduce the response time, of radiation measuring instrumentsusing an ionization chamber has been limited by the fact that thefeedback voltage can never completely; cancelout the potential developedacross the ionization resistor, since the device would then havenosignalfvoltage. This difiiculty hasbeen eliminated, as explained above,by the use of positive feedback developed in the input circuit of theamplifier, but exterior to the ionizationchambercircuit. The presentinvention differs from the. device covered by Patent No. 2,465,938,issued toiF'rancis R. Shonka, in'that the voltage fed back to reduce theresponse time of the ionization chamber circuit also appears in theinput circuit ofthea'mpli fier, while the device of theShonk'a patentfeeds back a voltage to the ionization chamber circuit which does notappear in the input circuit of the amplifier. Hence, the presentinvention achieves the advantages derived from the use of negativefeedback amplifiers, including greater amplifier stability, andminimizes the disadvantages inherentin suchnegative feedback amplifiers.i'

The man skilled inthe art will readily devise many equivalent methodsand means otherthan those illustrated in the drawing and describedspecifically above. The. invention accordingly is not limited to themeans and methodsspecificall'y described and illustrated, but panyingclaims.

Whatisclaimedisz I I 1. An instrument for the measurement ofparticlesand radiations, said instrument including an ionization chambercircuitprovidedwith' a voltage supply, amplifying means having an in-.

put circuit connected in series with a'portionof said ionization chambercircuitand'having an output circuit producing an output voltagevarye.

ing with the amount of ionization occurring in said ionization chamber,and meansto feed back a portion of said output voltage to the ionizationonly by the accommeans connecting the output circuit to the inputcircuit at a point exterior to the ionization chamber circuit.

2. Apparatus for the measurementof particles and radiations comprising,in combination: an ionization chamber circuit including in series avoltage supply, an electrical impedance, and an ionization chamber; adirect current amplifier including at least one vacuum tube, agrid-tocathode input circuit connected in series with the electricalimpedance, and a plate-to-cathode output circuit; negative feedbackmeans connecting the input and output circuits, said means including asecond electrical impedance connected in series with the ionizationchamber circuit; and positive feedback means connecting the outputcircuit of the amplifier to the input circuit at a point exterior to theionization chamber circuit.

3. Apparatus for the measuring of particles and radiations comprising,in combination: an

ionization chamber circuit including in series a voltage supply, anelectrical impedance, and an ionization chamber; and a two-stage directcurrent amplifier, the first stage including a vacuum tube, agrid-to-cathode input circuit connected in series with the electricalimpedance, and a plate-to-cathode output circuit including a second anda third impedance, the second of said impedances being connected inseries aiding relationship with the ionization chamber circuit, and thesecond stage of the amplifier including a vacuum tube, a grid-to-cathodeinput circuit connected in series with the second and third impedances,and a plate-to-cathode output circuit connected in series with the thirdof said impedances, the cathode of said vacuum tube beingdirectlyconnected to the cathode of the vacuum tube in the first stage.

4. Apparatus for the measurement of particles and radiations comprising,in combination: an

ionization chamber circuit including in series.

a voltage supply, a first electrical impedance, and an ionizationchamber; a one-stage direct current amplifier including a vacuum tube, aplateto-cathode output circuit, a second impedance connected in serieswith the output circuit, said second impedances being also connected inseries aiding relationship with the voltage supply in the ionizationchamber circuit, and a grid-to-cathode input circuit including both ofsaid electrical impedances; and means responsive to the output of saidamplifier to impress a voltage in series with the grid-to-cathode inputcircuit aiding the voltage developed across the first impedance, saidmeans being connected exterior to the ionization chamber circuit andbucking the voltage developed across the second of said impedances.

5. Apparatus for the measurement of particles and radiations comprisingthe apparatus recited in claim 4 wherein the means to impress a voltagein series with the grid-to-cathode input Oil?- cuit comprises, a thirdimpedance connected in series with the grid-to-cathode input circuit anddirectly connected to the cathode of the tube, a second direct currentamplifier stage including a vacuum tube, a grid-to-cathode inputcircuit, means to connect said input circuit to the output of the firstamplifier, and a plate-to-cathode output circuit connected in serieswith the third of said impedances.

6. Apparatus for the measurement of particles and radiations comprising,in combination, the apparatus recited in claim 5, and means to measurethe change in current through the output circuit of the second of saidamplifiers stages.

7. A radiation measuring device including an ionization chamber circuithaving an ionization chamber, a voltage source, and a high impedance,and an amplifier having a grid-tocathode input circuit connected inseries with the high impedance, said amplifier being provided withnegative feedback to the input circuit,

characterized by the improved construction comprising a second impedanceconnected in series with the input circuit of the amplifier exterior tothe ionization chamber circuit, and means to create a positive feedbackvoltage across said second impedance.

HOWARD V. HERNDON. RODNEY G. HOFF.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES MDDC-833, Jan. 17, 1947;AECD-2051, Wu and Rainwater, May 25, 1945. AECD-205l, Wu and Rainwater,June 8, 1948.

